Aluminum alloy, featuring excellent specific strength, specific stiffness and corrosion resistance, has been one of primary structural materials for aerospace equipment such as a rocket and an aircraft. The aluminum alloy accounts for about 80% of the structural mass of a carrier rocket and above 50% of the structural mass of a civil aircraft. With the development of a new generation of large rockets and aircrafts, an urgent need emerges for large-sized integral structure comprising aluminum alloy thin-walled components to meet their requirements for higher reliability, longer lifespan and lighter weight.
An existing technical roadmap for manufacturing aluminum alloy thin-walled component was presented as “sheet metal forming separately, welding into an integral component and heat treatment for property control” in one prior art literature. The prior art has the main problems that a relatively high degree of distortion is caused after welding, and an even greater distortion is caused after the heat treatment. What's more, the integral thin-walled component can't be subjected to shape correction after forming and welding, and the prior art method usually leads to lower precision and a failure to meet the use requirements. In order to solve the problems above, a technical roadmap to be adopted is “sheet metal tailor-welding for preparing a large-size tailor-welded plate, heat treatment for property control and integral forming using the large-size tailor-welded plate for a large-size thin-walled component”. For the advantage of high strength coefficient of weld joint, friction stir welding (FSW) has become a preferred welding method for aluminum alloy components in the aerospace field in recent years, instead of fusion welding methods such as arc welding and laser welding. Therefore, there is an urgent need for development of a large-size integral component forming technology using aluminum alloy FSW tailor-welded plate.
However, there are some insuperable difficulties for forming the larger-sized aluminum alloy thin-walled integral component by an existing conventional cold forming (forming at room temperature) technology and a hot forming (forming at elevated temperature) technology. As to the cold forming technology, a larger-sized thin-walled tailor blank is prone to wrinkle and a FSW weld joint is prone to crack when a conventional deep drawing technique is adopted, thus both the wrinkling and cracking defects exist and can't be overcome. Sheet hydroforming has been looked as a promising cold forming technology for large-size thin-walled component with curved surface. However, the forming force of a component with the diameter of 5 m reaches 800 MN, and the cost and risk of super-large fluid high pressure forming equipment are extremely high when sheet hydroforming technique is adopted. As to the hot forming technology, the FSW weld joint is softened in heating status, and the cracking problem can't be solved for the lower strength caused by softened weld joint in the forming process. Furthermore, there are very difficult to control the microstructure and mechanical properties of the formed component in the hot forming process.
In order to solve the problems when the larger-sized aluminum alloy integral thin-walled component is manufactured with the traditional forming technologies, a method called frozen forming technology is invented for forming of larger-sized aluminum alloy tailor-welded component at very low temperature by utilizing a new phenomenon that the aluminum alloy sheet is enhanced both on plasticity and strength at a very low temperature as described herein below.